The present invention relates to the field of non-invasive devices for analysis by radio-imaging.
Numerous analysis devices have already been proposed which use radioactive markers (Geiger counters coupled to a collimator, gamma cameras, etc.).
By utilizing the inherent property of radioactive marking which makes it possible to obtain quantitative information regarding the distribution of the tracer, radio-imaging techniques constitute an important tool both in the clinical field and in the field of fundamental research.
At present, the devices which are used most widely in this field belong to computer-aided emission tomography.
Computer-aided tomography has been developed along two different lines: the SPECT (Single Photon Emission Computed Tomography), which uses radio isotopes emitting a single photon by decay, for example 99mTc, and the PET (Positron Emission Tomography) system, which uses radio isotopes in which two gamma radiation events are emitted simultaneously during the annihilation, in the tissue, of the positron produced by the decay of, for example, 18F.
Most SPECT systems are based on the use of one or more gamma cameras which are rotated about the object to be analyzed. A typical gamma camera consists of a multi-channel collimator, a large-area scintillator crystal, a light guide for optical coupling between the crystal and a set of photomultiplier tubes, and analog electronics for analyzing the amplitude of the signal and the position encoding. The entire device is contained within lead shielding in order to minimize the background noise produced by sources lying outside the field of view of the camera. The operating principle of a gamma camera is as follows: a photon, produced by a decay event in the source and passing through the collimator, can interact with the scintillator, provoking a local and isotropic scintillation. The photomultiplier tubes located above each receive a light flux which depends on their distance from the light source. It is then possible, on the basis of the electrical signals delivered by each photomultiplier, to reconstruct the position of the scintillation by a center-of-gravity technique and to record and/or send it to a display device.
Positron emission tomography (PET) is another method which makes it possible to achieve in vivo and non-invasive regional measurement of physiological and metabolic parameters. Positron-emitter radioelements are isotopes having a surplus of protons with respect to their number of neutrons. When a positron is almost at rest, an encounter with an electron gives rise to an annihilation reaction which produces the simultaneous emission of two gamma photons departing in almost opposite directions. PET systems thus comprise an array of detectors in a ring which can detect the coincidence of two photons, as being indicative of the emission of the positron. The site of the annihilation then lies somewhere in the volume defined between the two detectors in question.
U.S. Pat. No. 4,288,697 describes a collimator formed by a stack of plates which are provided with perforations that correspond to a homothetic progression and are produced by chemical machining.
IEEE Transactions on Nuclear Science, Vol. 41, no. 4, describes a conventional PET structure without a focusing collimating structure.
EP-A-0 289 737 describes a conventional scanner having a focal point.
Radio-pharmaceutical imaging constitutes an important tool in the diagnosis, characterization and treatment of diseases and functional disorders. However, before new pharmacological agents are used in man, it is generally necessary to characterize them in animal models in order to determine its biochemical, metabolic and physiological effects.
Of course, this characterization presupposes the availability of high-resolution imaging techniques in order to evaluate, ex vivo or in vivo, the spatial concentrations of the tracer which is injected.
At the present time, the spatial resolution of conventional tomographs is from 5 to 7 mm in the case of PET systems and from 8 to 12 mm in the case of SPECT systems. These values prove to be insufficient for carrying out studies in small animals, for example for rat studies of tumors, the typical size of which is of a few mm, or the distribution of neuroreceptors. In actual fact, it is necessary for a tomograph dedicated to the imaging of small animals to be able to provide spatial resolutions of at least xcx9c2 mm.
Since 1990, a number of approaches based on PET and SPECT systems have been pursued in an attempt to achieve the desired performance.
However, these attempts at improvement have not yet been satisfactory, except at the cost of detection efficiency. The limitations of current tomographs in terms of resolution do not therefore allow in vivo studies to be extended to models on small animals, for which experimentation could be carried out more precisely.
The object of the present invention is to improve this situation.
This object is achieved according to the present invention by virtue of an analysis device comprising a plurality of detectors which are associated with collimating structures having a common source focus and detector output processing means, characterized in that the processing means perform a combinatorial logic function of the xe2x80x9cANDxe2x80x9d type on the output of the detectors in order to detect two coincidentally emitted radiation events that are at least slightly angularly correlated.
According to another characteristic of the invention, a multi-channel collimator is provided which is formed by a stack of plates having perforations, the thickness of the plates is less than the diameter of the perforations in the internal entry face of the collimator and the thickness of the span between the perforations is greater than the thickness of the plates.
According to an advantageous characteristic of the invention, the perforations in the plates are produced by chemical machining.
The present invention also relates to an analysis method which comprises the steps consisting in:
injecting, into a body to be analyzed, a marker which can generate two coincidentally emitted radiation events that are at least slightly angularly correlated, and
detecting these radiation events using a device of the aforementioned type.